April 2008 Technical Meeting
By Gerard Hillenbrand, P.E.
Met section’s April technical meeting was held on Thursday, April 17th, 2008 at Con Edison Head Quarters on Irvington Place and 14th street in Manhattan. The topic of the meeting was the “technique and applications for subsurface “technique and applications for subsurface imaging using ground penetration radar.” The meeting’s speaker was John D. Ciampa, director of geophysical services & environmental remediation for the spectra subsurface imaging organization, LLC, of Latham, N.Y spectra’s services are widely used by con Edison and other utilities to investigate underground conditions at construction sites throughout the unite states professional engineers attending this meeting qualified for 1.5 hours of credit toward to the continuing education requirements mandated a few years ago by the N.Y. state education department.
Underground imaging techniques are not new developments but recent advanced computer and data processing methods have optimized these systems to provide precise subsurface mapping that is accurate and complete. The imaging techniques employ electromagnetic radiation devices emitting frequencies approximating those used in microwave cooking appliances. Various frequencies are used depending on the depth of ground penetration investigated. For example, a frequency of 100 micro hertz will detect obstructions at depths of 50 feet, 200 MHz at 25 ft., 400 MHz at 10 ft., 1000 MHz at about 4 ft., and 1500 MHz between the surface and depths of 3 feet. The generated frequencies are dependent on the type, orientation and design of the antennas employed. There are two (2) basic types of antennas – the ground coupled type and the air launched type which is capable of deeper ground penetration but with reduced image resolution. The air launched antenna is also more mobile and provides more flexible operation. The antenna emitted electromagnetic waves are reflected off underground object and detected by a separate receiving antenna coupled to a data recorder for processing and interpretation. Different Buried materials have different electromagnetic reflective properties and these differences provide the means for accurate identification.
The antenna configurations and data collection devices are usually mounted on a moving conveyance similar to a bicycle or golf cart and uniformly driven over the ground area requiring investigation. Buried objects reflect the electromagnetic waves as previously described. The speed of the conveyance is, of course, variable and adjusted continually since the time required to detect the reflected wave is proportional to the depth of the buried object. For example, a round buried pipe or conduit would produce a reflected wave depicted as an inverted hyperbola on the data recording screen. Other obstructions and various soil conditions all have various image characteristics and interpretation of these images requires a good deal of practical experience and exposure. This is one skill that is usually not taught in engineering schools.
Various soils are frequently layered configurations of differing materials and, since each material has differing reflective properties, the interfaces of these differing soil materials can be visually defined in the resulting image, as can material voids also. For example, soils having a high clay content have relatively poor reflective properties, but soils with high salt content have good reflective properties. The amount of moisture in the soil can also affect reflective properties, as can the presence of multiple objects, the size of the objects and their depth below the surface. The resulting images can be displayed on a personal computer, providing opportunities for rapid analysis. Antenna controls provide adjustable amplitudes for the emitted signals along with signal velocity control, as well as filtering circuitry and stacking provisions for multiple signals.
As such, these ground penetrating radar techniques provide safe, non-destructive methods for discovering buried objects along with rapid, continuous, and large volume data collection with minimum service reduction and construction delays. Units with single channel antennas can survey areas of 1500 square feet per hour at a cost of about $4000 per day. Multi-channel units (with as many 14 channels emitting 400 micro hertz) can survey areas as large as 40,000 square feet in one working shift at cost of $0.50 to $0.75 per square foot. Recent developments include hand-held units weighting no more than two pounds, used for investigating the integrity of masonry walls and floors. Data collection units now can provide printouts in the field as well as CAD compatible photographs. Incidentally, all antenna designs and configurations must be approved by the federal communications commission to eliminate any potential interference with other media.
Mr. Ciampa’s computerized slide presentation then continued with illustrations of successfully completed projects on infrastructure inspections, utility piping maps, geological and geotechnical investigations, building foundation testing, geophysical and archeological searches, and concrete pavement and structure tests varying from several inches to 50 feet in depth. Among the specific case studies where:_
- In Yonkers, a utility construction site 300 ft long x 40 ft wide was scanned to locate buried utility and electrical lines from depth of 1 inch to 10 feet. The results were used to locate proposed vault which consolidated the complex system of existing lines.
- In Manhattan, A similar study produced a series of depth slices or cross-section views which discovered the location of buried rail and trolley tracks. Investigations in times square also discovered the locations of various buried steel beams and wire mesh as well as an abandoned concrete vault.
- In upstate New York, a geological mapping study was conducted in an abandoned quarry to locate the paths of water leaks from an adjacent reservoir. Several sealed mines, earth voids and rock fracture zones were located and a drilling program was developed through which grout was pumped to seal the vacancies. The result was a 90% reduction in water loss.
- At a superfund site, tests were conducted to locate a lost, buried tank and to optimize the location of proposed water line. The tests discovered bedrock as close to 10 feet of a surface.
- At an abandoned cemetery, a subsurface radar investigation varying from depths of 1 to 6 feet located sunken tombstones dating from the 1930’s, crushed stone pathways, and a ,multiple number of graves, all located in a vacant city lot.
- At locations which are confidential, subsurface imaging techniques are employed to locate and quantify sties for future petroleum extraction, investigations which can result in potentially valuable additions to current supply.
The usual question and answer period followed Mr. Ciampa’s presentation. Several engineers present exchanged business cards with Mr. Ciampa to follow up with the possibility of further study covering their own technical requirements. Licensed professional engineers who attended this technical lecture qualified for 1.5 hours of professional development credit. This technical lecture was typical of those periodically presented by ASME’s metropolitan section to enable our member mechanical engineers to remain abreast of the latest technological advances. We look forward to your attendance at our future meetings.